Theoretical and experimental aspects of PIV recording utilizing photographic film and mechanical image shifting

Kurzfassung

The current level of technology allows photographic recording to achieve a higher spatial resolution than can be obtained with current methods of digital video recording, especially if using large formats. Though CCD sensors with increasing resolutions are continuously under development and brought into the market, it will be some time until video recording techniques can match the spatial resolution of present day film material. Another problem associated with video PIV is the requirement to store the huge amount of data, which may be captured during a measuring campaign. Thus, for time being, the photographic technique is the method of choice for PIV applications requiring high velocity and spatial resolution. One major disadvantage of the photographic technique is that - at present - it is difficult to record the images of the tracer particles onto two different photographic plates, especially in the case of aerodynamic investigations where pulse separations on the order of a few microseconds are required. That indicates that the problem of directional ambiguity removal has to be solved for photographic PIV in a reliable and flexible manner using a technique such as image shifting. Additionally, the application of an image shift to the PIV recording allows PIV measurements in strongly three-dimensional flows as well as in flows with strong velocity gradients. By 'negative' image shifting the resolution of the velocity measurement can be increased with a slight reduction to the spatial resolution. A rotating mirror system, which is able to provide very high and easily adjustable shift velocities, will be describled in the following. Problems such as the angular control of the rotating mirror and its synchronization with the laser pulses have been solved. A detailed theoretical model of the imaging of tracer particles by means of a rotating mirror system has been developed in order to be able to correct certain errors appearing at imaging via a rotating mirror